Watch City Steampunk Festival
This free event in downtown Waltham draws 10,000 people. Activities include craft vendors, cosplay, live music and more. The 2019 festival will include a "Mad Science Exhibition." From the website: "On festival day 2019, makers, inventors, artists, engineers, hobbyists, and crackpots of all sorts will exhibit their most astonishing creations on Waltham Common. The most impressive among them will be awarded prizes!"
The address for GPS is Waltham City Hall at 610 Main Street.